1. Field of the Invention
The present invention relates to a differential transmission circuit, and an optical module and an information processing system using the same, and more particularly to improvement in quality of a transmission signal transmitted on the differential transmission line.
2. Description of the Related Art
A transmission line is used for high-speed digital signal transmission. For example, differential transmission is generally used for digital signal transmission of several hundreds of Mbps or more instead of single-end transmission, and a transmission line for performing the differential transmission is a differential transmission line. The differential transmission line is formed by a ground conductor layer, and a pair of (two) transmission line conductors provided over the ground conductor layer via a dielectric layer, extending in a strip shape, and formed in the same layer. The differential transmission is advantageous in that the amplitude of a signal voltage is made to be small, and thus influence of noise hardly exerts, whereas it is disadvantageous in that transmitted differential signals is deteriorated if the lengths of a pair of transmission line conductors are different from each other. This is because a delay time difference is generated in two differential signals transmitted on the differential transmission line if the lengths of a pair of transmission line conductors are different from each other. Therefore, a pair of transmission line conductors are generally formed to be parallel to each other and to extend in a straight line shape such that the lengths of a pair of transmission line conductors are the same as each other. However, there are cases where it is necessary for the transmission line to be disposed so as to be bent depending on positional relationship or the like with elements disposed on a printed circuit board.
FIG. 26 is a plan view illustrating an example of the differential transmission line according to a related art. The differential transmission line shown in FIG. 26 includes a bent region which is bent by 90 degrees and a straight line region which extends in parallel. FIG. 26 shows a P data conductor film 181 and an N data conductor film 281 as a pair of transmission line conductors, and, in the bent region, the length of the P data conductor film 181 passing the outside is larger than the length of the N data conductor film 281 passing the inside. Therefore, a delay time difference occurs in two differential signals transmitted on the differential transmission line.
As related arts for compensating for the length difference of a pair of transmission line conductors, due to the presence of the bent region, there are the two following techniques. One is a technique where the lengths of two conductor films are the same as each other through examination of the shapes of the P data conductor film 181 and the N data conductor film 281, thereby achieving equal length wires. That is to say, the N data conductive film 281 which has a shorter length due to the bent region has a shape which is partially meandered in other regions, so as to reduce the length difference of a pair of transmission line conductors, thereby compensating for the delay time difference occurring in two differential signals.
The other is a technique where plural slots are provided on the ground conductor layer in the bent region of the differential transmission line, and the technique is disclosed in Japanese Patent No. 3954641. In the bent region of the differential transmission line, plural slots are provided on the ground conductor layer which is located under a pair of transmission line conductors, and thereby a high frequency current flowing through the ground conductor layer corresponding to the transmission line conductor with the smaller length circumvents the slots so as to compensate for the delay time difference of two differential signals occurring due to the length difference of a pair of transmission line conductors.